Magnetic fluctuations embedded in dipolarization inside geosynchronous orbit and their associated selective acceleration of O⁺ ions

Abstract

We study magnetic fluctuations embedded in dipolarizations in the inner magnetosphere (a geocentric distance of ≤6.6 R E ) and their associated ion flux changes, using the Engineering Test Satellite VIII and Active Magnetospheric Particle Tracer Explorers/CCE satellites. We select seven events of dipolarization that occur during the main phase of magnetic storms having a minimum value of the Dst index less than −40 nT. It is found that (1) all of the dipolarization events are accompanied by strong magnetic fluctuations with the major frequency close to the local O + gyrofrequency; (2) the magnetic fluctuations appear with significant amplitude in the component nearly parallel to the local magnetic field; (3) the strong flux enhancement is seen in the energy range of 1-10 keV only for O + ions. In terms of frequency and dominant components of the magnetic fluctuations, they are considered to be excited by the drift-driven electromagnetic ion cyclotron (EMIC) instability that is recently identified with the linear theory. We perform particle tracing for H + and O + ions in the electromagnetic fields modeled by the linear dispersion relation of the drift-driven EMIC instability. Results show that the O + ions are accelerated to the energy range of 0.5-5 keV and undergo a significant modification of the spectral shape, while the H + ions have no clear change of spectral shape, being consistent with the observations. We therefore suggest that the electromagnetic fluctuations associated with the dipolarizations can accelerate O + ions locally and nonadiabatically in the inner magnetosphere. This selective acceleration of O + ions may play a role in enhancing the O + energy density in the storm time ring current.

Ion Energies Dominating Energy Density in the Inner Magnetosphere: Spatial Distributions and Composition, Observed by Arase/MEP‐i

“…Mass-dependent acceleration may be possible in the plasma sheet with magnetic and electric field perturbations (e.g., Catapano et al, 2016), in the reconnection region and around the separatrix between closed and open magnetic fields (where the Hall electric field exists; Liang et al, 2017), around a dipolarization front (i.e., a reconnection jet front) where the magnetic field increases in less than seconds and a strong electric field is thus induced (Runov et al, 2015), near the transition region from the stretched magnetic field to the dipole-like field where a fast flow associated with reconnection slows down and the electric field is induced by the magnetic field pileup (Nakayama et al, 2016). Possible waves are dispersive Alfvén waves (e.g., Chaston et al, 2016), which can accelerate O+ that are simultaneously extracted from the topside ionosphere and/or preexist on the same field lines; waves/fluctuations with a frequency range of electron-ion cyclotron waves (e.g., Nosé et al, 2014); and ULF waves (e.g., Mitani et al, 2018;Oimatsu et al, 2018), which can resonate with drifting and bouncing ring current ions. However, as higher-energy O+ are more difficult to transport to the inner magnetosphere, it is still a controversial issue whether a large number of those accelerated O+ can penetrate to around/inside geosynchronous orbit.…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…There has been a recent suggestion that the thermal O + torus can be accelerated by nonadiabatic mechanisms to ring current energies . Nosé et al (2014), showed that the magnetic fluctuations associated with dipolarizations inside 6.6 R E have a frequency close to the O + gyrofrequency, and can preferentially accelerate the O + to 5-10 keV. The population from 1-10 keV can also drift in from the nightside plasma sheet, so the importance of the energization is not clear.…”

Storm time impulsive enhancements of energetic oxygen due to adiabatic acceleration of preexisting warm oxygen in the inner magnetosphere

“…Even around the time of the proton injection event at ~20:00 UT, no sharp increase in the z component was seen. The B z profile is different from typical dipolarization events observed in the inner magnetosphere [ Nosé et al ., , ]. The proton injection was slightly energy‐dispersed as shown in Figure .…”